A wide variety of backrests are known. Most are inherently rigid. A hard shell is provided only with upholstered padding on the seat side.
Ergonomic seating however requires flexible resilience corresponding to the body shape supported by the supporting element in each case. In particular due to the different sizes and seat positions flexible resilience corresponding to the back shape supported by the seat-back as well as permanent support of the pelvis boundary is required in order to prevent the pelvis from tilting to the rear.
The unclear text of the published EP-A-1040999, which is based on the similarly unclear application DE-A-19916 411, discloses a component for the absorption of forces, which component in the linear direction possesses a blunt and a pointed end and a flexible outer skin, which covers the component from the blunt to the pointed end on two sides. The component is to be fitted at the blunt end, while the pointed end should project freely. The outer skin forms a coherent, integral unit on the lower face and topside of the component. Lower face and topside are interlinked by cross struts. Connecting devices for the cross struts are formed on the inside of the outer skin. The cross struts are hinged in these connecting devices. Owing to the parallel aligned cross struts the flexible and dimensionally-stable outer skin is kept to a deformable profile. This structure of the component ensures that it moves against a force acting upon the outer skin. In this publication it is proposed that such a component can be fitted in chair-backs or seat areas. By connecting two frames (such a component should eventually be described as a frame), whose blunt ends are connected to one another by a hinge, a chair is created, which is able to seat a person and adjust to the anatomy of that person. FIG. 20, which as a single drawing illustrates a chair, shows an upholstered seat and a backrest, that are both designated with the reference number for one component. These two components are joined on a common pin and apparently flexibly held in a position relative to one another by a spring.
This chair concept was refined in the US Patent US-A 2004/0183348. This publication discloses a support element corresponding to the afore-described component, that possesses a skeleton, said skeleton exhibiting a skin, to which a plurality of ribs are attached. The skin forms a flexible support area for supporting a seating force, exerted by a body on the skin. This skeleton cooperates so as to at least partially deform said support area in a direction opposite the seating force as a result of said seating force. The skeleton further comprises at least one biasing element, which couples together skin and/or ribs, or the skin forms a backrest and a seat area as a unitary piece. The biasing action of the biasing element in particular causes the shape of the supporting element to be adjusted. For this purpose the biasing element is arranged in a diagonal of the rectangle, which consists of two ribs and the skin arranged on both ends of the ribs.